In the processing of materials containing metal values, two of the main extractive methods to be considered are pyrometallurgy and hydrometallurgy. In the former, metal-containing material such as ore, slag, scrap, etc., is heated with appropriate agents such as reducing agents, fluxing agents, sulfidizing agents, chloridizing agents and/or oxidizing agents, etc., usually to the melting or fusion point of the mixture. At this temperature there is generally a separation of metallic values from gangue or waste materials. The procedure then calls for separating the metallic values from slag or waste material at a temperature at which both are molten. The phase containing the metal value is then cast to some convenient shape for use or for further refining, whichever is appropriate for the particular system involved. The very high temperatures involved in this technique are achieved via electric furnaces, blast furnaces, reverberatory furnaces, etc. Temperatures required for metals such as copper, nickel, iron would generally range from 1000.degree. C. to 2000.degree. C. An advantage in this method is that recoveries of the metal values are typically quite high.
The hydrometallurgy approach differs substantially from pyrometallurgy in that, although the metal bearing material such as ore, slag, scap, etc., may be heated with agents such as reducing agents, oxidizing agents, sulfidizing and chloridizing agents as part of the procedure, the temperatures involved are generally much lower than with the usual pyrometallurgical method. These temperatures typically may be 260.degree. C. to about 1040.degree. C., temperatures generally well below the fusion point of the metal-containing material.
Following this step, the treated metal-containing material then is contacted with an appropriate aqueous solution for extracting metal values by dissolution. The metal is then removed from the solution by precipitation, solvent extraction, evaporation of solvent, etc. The metal-containing residue obtained is then handled appropriately to further refine the metal. Although conditions of temperature are generally much lower than in pyrometallurgy, it is frequently found that recovery of the metal values is also lower than in the pyrometallurgical method.
A particular case where this is true concerns the extraction of nickel from lateritic nickel ores. The pyrometallurgical processes range from the use of an electric furnace for the direct smelting of ore to produce ferronickel through similar techniques involving the blast furnace in which an iron-nickel-sulfide matter is obtained. The extraction of nickel from the ore using this method is greater than 90%.
Of the several hydrometallurgy approaches used commercially for treating this type of ore, the practice on a limonite ore or a highly serpertinic ore, such as that at Nicaro, Cuba, involves roasting the ore in a multihearth furnace while a reducing gas, such as producer gas, passes countercurrent to the ore. Temperatures in this case range from about 485.degree. C. to about 735.degree. C. Following the roasting step, the ore is cooled in the absence of air, mixed with an ammoniacal ammonium carbonate solution and vigorously agitated and aerated. This results in the dissolution of nickel, copper, and cobalt, separating them from the bulk of the core. This solution then is treated with steam, driving off ammonia and precipitating nickel carbonate. This product then is treated further to obtain the appropriate form of nickel or use as such. In comparison to the pyrometallurgical process, however, extractions using this method have only been of the order of 70 to 80 percent.
Several other hydrometallurgy methods involve the use of procedures which include a roasting step with chlorides or sulfates but in other than reducing atmospheres and in such a manner as to form soluble metal salts, and the roasted ore is leached with an appropriate solvent such as dilute sulfuric acid. Alternatively, in certain cases the ore can be leached directly, such as with sulfuric acid solution but this is practical only when the magnesia content of the ore is low.
The extraction of metal values from metal bearing sources may be improved when the reductive roast is effected in the presence of certain additives such as added halides, added sulfur, added sulfur-containing compounds or combinations of these additives. The addition of these additives in which the metal bearing source is pretreated with the additive is known in the trade as "pugging" step. Normally, when utilizing a halide, the pugging step introduces a large amount of water into the process. As will hereinafter be shown, it has now been discovered that only a portion of the metal bearing source which is the feed for the process need be contacted with the required amount of additive thereby permitting a sizable reduction of the amount of water which is required and concurrently permitting conservation of energy and the use of less expensive corrosion resistant equipment. One means of accomplishing this is to utilize a portion of the ore feed as a dry scrubbing medium for the off-gases from the reductive roast of the ore.
U.S. Pat. No. 3,661,564 discloses a chloridizing roast at temperatures ranging from about 250.degree. to about 350.degree. C. in which the patentee adds a sufficient amount of chlorides such as hydrochloric acid to a minor portion of a lateritic ore so that when the minor portion is added to the major portion of the ore there is enough total chloride present to totally chlorinate the metal values of interest such as the nickel, copper, and cobalt and to form soluble nickel chloride, copper chloride, and cobalt chlorides. The purpose of this procedure is to permit the extraction of those metal values from the ore by a simple aqueous leach. This is in contradistinction to the object of the present invention to which the minor portion of the feed when treated with at least one additive and combined with a major portion of the feed is reductively roasted at a temperature which is sufficient to destroy any soluble salts which might have been formed. In addition, the procedure also increases eventual metal value recovery and can be employed on a wider range of ore compositions.